1. Field of the Invention
The present invention relates to an image reader comprising a solid-state image sensor for reading the density of an original and, especially, relates to the adjustment for the reading magnification and focusing of the image of the original.
2. Description of the Prior Art
In an image reader of this type, the density of an image is read by a solid-state image sensor usually consisting of a linear array of micro cells each of which is made as a charge coupled device (CCD) and is output as binary signals obtained by processing outputs of the solid-state image sensor.
In an image reader including a zooming mechanism for changing the magnification of an image optically, the adjustment for the magnification and focusing is done by displacing the focusing lens and/or the solid-state image sensor. It becomes necessarsy to set the focusing lens and the solid-state image sensor respectively at positions to be determined by the magnification designated when the power switch was turned on, or when a new magnification was designated by an operator.
In a conventional image reader, the focusing lens and image sensor are reset to such a respective normal position that an equal magnification is obtained thereat when the power switch was turned on.
When a new magnification was designated, the focusing and magnification are adjusted in turn. Namely, for instance, the focusing is adjusted at first and, then, the magnification is calculated in that state and, thereafter, the focusing lens is displaced so as to provide the magnification designated. However, since the focusing and magnification are correlated with each other, the magnification is slightly changed due to the adjustment of the focusing even when the focusing lens has been set to a position determined from the magnification designated prior thereto. Accordingly, it becomes necessary to repeat the adjustment of the focusing and magnification until the designated magnification is obtained in a fine focusing state.
Furthermore, the focusing lens used in the image reader is designed as a contractive optical lens system usually, since it is necessary to project the original image onto the image sensor having a smaller dimension than that of the image.
Due to this, the depth of field of the focusing lens becomes very shallow to an order of .+-.0.01 mm. Therefore, it is very difficult to set the focusing lens mechanically at an exact focusing position.
In order to control the position of the focusing lens exactly, there has been proposed a focusing system being characterized in that a sheet on which a pattern of black and white vertical stripes is printed is provided for adjustment for focusing, that, upon focusing, said stripe pattern is made projected onto the image sensor through the contraction lens system in order to cause an interference between the projected stripe pattern and the pitch of the cell of the image sensor and that the position of the focusing lens is controlled so that the contrast obtained from the outputs of the image sensor become a maximum (Japanese Laid-Open Publication No. 173705/1983).
However, this proposed focusing system has a disadvantage in that the modulation transfer function (hereinafter referred to as M.T.F) is lowered considerably at each edge portion of the angle of field, since the position for focusing control is set at the center of the angle of the field. Further, it has another disadvantage in that the focusing control becomes unstable in such a case that where no interference by the stripe pattern can be obtained when a considerably large or small magnification was designated.